Wireless power transfer (WPT) system is a technology of transferring power through a space without any wire, which maximizes ease of providing mobile equipment and digital appliances with power.
The wireless power transfer system has advantages of saving energy by a power usage control in real time, overcoming a restriction of space needed for providing power and reducing waste batteries by recharging batteries.
The wireless power transfer system is typically embodied in a magnetic induction scheme or a magnetic resonance scheme.
The magnetic induction scheme is a non-contact energy transfer technique where current is applied to one of two coils disposed closely each other and by means of the magnetic flux generated accordingly electromotive force is also applied to the other coil, which may use frequencies of hundreds of kHz.
The magnetic resonance scheme is a magnetic resonance technique where electric field or magnetic field only is used without using electromagnetic waves or current so that the distance of power transfer is more than several meters, which has a characteristic that band having tens of MHz is used.
However, there is a problem that power loss occurs due to the current loss at a receiving side.
Additionally, in the wireless power transfer system, there occurs a spurious wave, that is, a harmonic coming from a power transmitter, and such a harmonic component may cause an electromagnetic interference effect, thereby having a harmful effect on a body.
The harmonic refers to a frequency higher than a commercial frequency in terms of power, hundreds of Hz or higher, for example, which has integer multiple frequency of a fundamental.
The wireless power transfer system generates the harmonic component near the multiple frequency of the receiving frequency due to a non-linear characteristic of the receiver load, basically. That is, the AC signal provided from the receiver does not remain as its waveform but is distorted due to the non-linear load, forming a harmonic.
The harmonic component disturbs normal operations of surrounding equipment or causes undesired power reception, noise and various obstacles.
In more detail, changes of magnetic field emitting from the power transmitter generate current caused by electromagnetic induction phenomenon in a conductor of a stationary surrounding equipment, capable of causing abnormal operations. Also, magnetic field generated in the power transmitter causes current in a mobile equipment when the mobile equipment moves around the magnetic field, capable of causing obstacles. Also, when the power transmitter has a resonance condition similar to that of the surrounding equipment that does not want to receive power, there may occur a magnetic resonance coupling, capable of causing erroneous operations in the surrounding equipment.
Since the harmonic component may cause harmful effect on an electronic equipment and a body as described above, it is important to meet regulations for an electromagnetic compatibility (EMC), an electromagnetic interference (EMI) and an electromagnetic susceptibility (EMS).
FIG. 1 is a block diagram of a transmitter for transmitting power in a conventional wireless power transfer system.
Referring to FIG. 1, the transmitter 1 may include a gate driver 2, a power converter 3 of a half bridge type, a matching circuit 4, and a transmission coil 5.
A half bridge circuit of the power converter 3 in the art has two switches that are complementarily turned on and turned off when proper voltage waveforms are applied to gage driving inputs, respectively.
At this time, a square wave voltage generates which is used to switch between a common node of the two switches and the ground.
Since the output not of a sine wave but of the square wave has a problem that it has many of harmonic components that are integer multiple frequency of a fundamental wave, it is limited to reduce the harmonic using the power converter 3 in the art.